The present invention relates to a thermal type flowmeter which is useful particularly, but not exclusively, for measuring the amount of air input to an internal combustion engine.
The Japanese Patent Provisional Publication No. 55-43447 discloses a conventional thermal type flowmeter in which a thermo-sensitive resistor (temperature sensing device) is provided within a fluid passage so that the flow rate in the fluid passage is measured based on the amount of energy supplied (electricity supply amount) to the thermo-sensitive resistor. The heating value of the thermo-sensitive resistor is then adjusted to cause the temperature of the thermo-sensitive resistor to become constant. Here, an operational amplifier is generally used for the adjustment of the amount of energy supplied to the thermo-sensitive resistor.
FIGS. 11 and 12 show a typical circuit arrangement of a conventional thermal type flowmeter. In FIG. 11, two thermo-sensitive resistors 51 and 52 together with resistors 53 to 55 constitute a bridge circuit where resistors 51 and 52 are to be provided in a flow passage. A current is supplied to the bridge circuit through transistors 56 and 57 darlington-coupled to each other. The voltages due to voltage dividing circuits of the bridge circuit are inputted to an operation amplifier 58, the output of which is inputted through a resistor 59 to the base of the transistor 57. Further, the voltage due to one of the voltage dividing circuits is inputted to an output circuit 60 which includes an amplifier, the output circuit 60 outputting a signal corresponding to the flow rate of the fluid based on that input. Moreover, an operating electric power is supplied from a battery 1 through a resistor 61 to both the operational amplifier 58 and the output circuit 60. Generally, a circuit as shown in FIG. 12 is provided for the outputting portion of the operational amplifier 60. In FIG. 12, the output of the operational amplifier 60 is derived through transistors 62, 63 which are provided at the positive side and darlington-coupled to each other. A transistor 64 is provided at the negative side. The bases of transistors 62 and 63 are coupled to the collector of the transistor 65 which is in turn coupled through a constant current source 67 to a power supply.
In such a conventional circuit, the minimum operating voltage V.sub.(MIN) necessary for supplying a current to the bridge circuit to measure the fluid flow rate becomes the sum of the voltage drops V.sub.BR, V.sub.BE1, V.sub.BE2, V.sub.2, V.sub.BE3, V.sub.BE4, V.sub.1 AND V.sub.CE(MIN) illustrated in FIGS. 11 and 12. Here, V.sub.CE(MIN) represents the minimum value of the voltage-drop voltage of the constant current source necessary for the constant current source which supplies the base current required to normally operate the transistors 62 and 63.
Although such a circuit is capable of measuring the flow rate in the fluid passage, it is not for measurement of systems having low supply voltages because the minimum operating voltage is relatively high. In addition, if using such a conventional circuit for a motor vehicle, difficulty is encountered when measuring the flow rate at the time of start of the internal combustion engine and when the battery voltage is lowered due to unsatisfactory charging to the battery. Particularly, because of these conditions, when a thermal type flowmeter is used for the measurement of the intake air quantity to the internal combustion engine, and the measured flow rate is used for the control of the fuel injection quantity to the engine, normal control of the fuel injection quantity to the engine becomes difficult.
Japanese Utility Model Provisional Publication No. 60-163316 and Japanese Patent Provisional Publication No. 55-43447 also disclose thermal type flowmeters. However, in the flowmeter circuit disclosed in the Japanese Utility Model Provisional Publication No. 60-163316, a transistor is provided in series with a bridge circuit including a thermo-sensitive resistor and voltage amplifying circuit. Hence, the voltage applied to the bridge circuit varies in accordance with the variation of the voltage of the power supply, whereby the signal voltage indicative of the flow rate also varies. Similarly, in the circuit disclosed in the Japanese Utility Model Provisional Publication 60-163316, an offset voltage obtained by dividing the supply voltage by a resistor is given to one input terminal of an operational amplifier. Thus, when voltage is supplied, it is applied virtually directly to a bridge circuit including a thermo-sensitive resistor, whereby there is the possibility that the thermo-sensitive resistor is damaged when the supply voltage is higher than a specified voltage. For example, in the case of using the flowmeter for the measurement of the intake air quantity to an internal combustion engine, an excessive voltage can be applied to the thermo-sensitive resistor during a jumper start (for example, a voltage of 24 V is given to a 12 V-motor vehicle to increase the rotation of the starter motor at the vehicle start time) which is often effected in cold districts.
Still further, the Japanese Patent Publication No. 63-43688 discloses a thermal type flowmeter where a voltage obtained by dividing the supply voltage by a resistor is applied through a diode to a differential amplifier to start a feedback control circuit. Additionally, Japanese Patent Provisional Publication No. 58-87419 discloses a thermal type flowmeter where a current is compulsorily bypassed with respect to the output of an operational amplifier of a heating-coil heating feedback circuit so as to start the control apparatus. However, both the prior art techniques require a terminal for inputting the start signal. Thus, noises may be similarly introduced into an input terminal of an operational amplifier, constituting a feedback circuit, at the normal operation time after the completion of start because a signal is given to the input terminal of the operational amplifier at the start time. Such noise may make it difficult to obtain an output indicative of an accurate flow rate.
In addition, the Japanese Patent Provisional Publication No. 55-43447 discloses a thermal type flowmeter which allows a stable flow rate measurement through two operational amplifiers: one is for controlling the energization amount to a heating thermo-sensitive resistor and the other is for controlling the amount of energy supplied to a temperature-compensating thermo-sensitive resistor. However, since this flowmeter circuit is arranged such that the current to the temperature-compensating thermo-sensitive resistor is supplied from the operational amplifier, the current flowing through the power circuit for the operational amplifier becomes problematically large. That is, when the current flowing through the power circuit becomes large, the voltage drop within the power circuit become great. Hence, the minimum operating voltage of the circuit operated through the power circuit becomes high, thereby making it difficult to measure the flow rate of the fluid when the supply voltage is low. Moreover, when the current flowing through the power circuit becomes large, a power circuit having a large capacity is required to allow the supply of a relatively large current, thus increasing the size of the circuit and cost thereof. In addition, for providing a small-sized and high-quality apparatus at a low cost, it is effective that the circuit is constructed with a monolithic IC. However, the power circuit having a large capacity to allow supply of a relatively large current requires a large area, thereby resulting in the increase in the size and cost of the IC chip.
Moreover, the Japanese Patent Provisional Publication No. 2-35315 discloses a thermal type flowmeter which is arranged to stably measure the flow rate with two differential amplifiers: one is for controlling the energization amount to a heating thermo-sensitive resistor and the other is for controlling the energization amount to a temperature-compensating thermo-sensitive resistor.
Further, this Japanese Patent Provisional Publication No. 2-35315 discloses a technique whereby a resistance bridge circuit including a thermo-sensitive resistor is constructed as a balancing bridge circuit. The connection to an operational amplifier is changed so that the effects of the internal offset voltage of the differential amplifier is reduced. According to this technique, when the internal offset voltages of the differential amplifier are set to V.sub.OS1 and V.sub.OS2, the total offset voltage for the entire circuit can be smaller than the sum of V.sub.OS1 and V.sub.OS2. As a result of reducing the total offset voltage, it is possible to improve the responsibility of the feedback control circuit. Although it is effective to reduce the total offset voltage of the feedback control circuit when trying to improve the response of the circuit, a positive offset voltage is required for stably operating the circuit. Thus, an offset voltage is positively given to one of the two differential amplifiers such as disclosed in the Japanese Patent Provisional Publication No. 3-15722. As described above, various attempts are taken in order to set the offset voltage to an appropriate value. However, variation in the internal offset voltage of the differential amplifier is inevitable and the internal offset voltage varies between differential amplifiers and further generally has a temperature drift of about 1 .mu.V/.degree. C. to 10 .mu.V/.degree. C. Accordingly, the total offset voltage including the internal offset voltage of the differential amplifier varies in accordance with the internal offset voltage of the differential amplifier. Hence, difficulties arise from circuits constructed using a differential amplifier which have an offset voltage set to a designed value. In addition, although it is considered that the offset voltage is measured after the construction of the circuit, and that the resistance value and the positively given offset voltage are adjusted so that a desired total offset voltage can be obtained, there is a problem that the adjustment increases the cost.